This multicenter study intends to establish the safety and efficacy of an oral osmotic dosage form of Ritalin (OROS). Children with ADHD will be screened and diagnosed (#9548A) and, not currently taking MPH, titrated to appropriate OROS dose (9548B). The experimental stage will be a randomized double-blind comparison of OROS, standard Ritalin, and placebo (9548C). Finally, a year of OROS treatment will be provided to evaluate long-term safety and efficacy (9548D).